Support additional context for route generation

[NiklasEi]

Using Leptos as a static site generator currently has two main steps:

1. generating routes
2. building static routes

The second step allows passing additional context (e.g. global state) via build_static_routes_with_additional_context that can then be used in components, but the first step doesn't. This means all components that get evaluated during step 1 will fail to get the additional context, while others that are only evaluated during 2 will be able to use it.

This PR adds support for additional context during step 1

[benwis]

Interesting, this definitely seems like a nice fix. I wonder why the actix check failed

[gbj]

The failing checks are all failing on clippy on leptos_reactive (or another leptos crate), I think because of some disagreement between Rust versions over whether the type_name_of_val feature is stabilized. Hopefully fixed in a4bd7c4

This looks good in any case. Thanks so much!